1. Field of the Invention
The present invention relates to an electro-magnetic nozzle device for use, in particular, at the outlet of a crucible to stabilize the tapping, at a variable flow rate, of a liquid metal in the form of ultra-clean material intended, in particular, for atomization to produce metallic powders such as for the manufacture of superalloy components for aeronautical applications.
The processes which are known and used at present for the production of superalloys, especially nickel-based superalloys such as those with which the invention is particularly concerned, involve melting operations in crucibles made of a ceramic type refractory material and performed under vacuum in a furnace. During such operations a metal/ceramic reaction occurs, which inevitably results in the presence of ceramic inclusions in the material obtained. A refining of the superalloy is accordingly necessary each time the conditions of use demand that a so-called super-clean superalloy should be obtained. This is particularly the case with nickel-based superalloys intended for aeronautical applications, such as components for gas turbine aero-engines or other propulsion units. Various known techniques are used to achieve such inclusion refining, e.g. by remelting in a cooled crucible, the melting being effected by electric arc, electron beam or plasma beam.
Whatever technique is used, however, when tapping the molten metal, whether to fill a mould or to atomize the liquid metal to obtain a powder, it becomes necessary either to swivel the furnace or to use a nozzle of refractory material at the outlet for the liquid metal. In the first case, controlling the rate of flow and the mass of the molten metal is virtually impossible, and, in the second case, although this problem is solved there are other drawbacks:
fairly large nozzle diameters are necessary so as to avoid the danger of clogging; PA0 instability of the jet of liquid metal; PA0 considerable difficulties in modifying the diameter of the liquid jet during operation. PA0 firstly, a chemical pollution due to the reaction of liquid metal at high temperature with oxides contained in the refractory materials from which the walls are made; and PA0 secondly, a physical pollution due to abrasion of the nozzle walls by the flow of molten metal.
Moreover, the contact between the liquid metal and the solid walls of the nozzle causes a double pollution of the metal:
In particular applications of known techniques of processing liquid metals by atomization with gas these pollutions lead to the presence of numerous inclusions in the metal powders, and it is recognized that the presence of such inclusions in rotary parts of aero-engines may be the source of faults in the service life of these parts, which are subjected to oligocyclic fatigue stresses, leading particularly to premature breakage when subjected to high stresses at high temperatures. These problems have led to reducing the grain size of the powders, resulting in very poor size grading efficiency in the production of these powders.
2. Summary of the Prior Art
Attempts at a solution have been proposed based on the use of an electromagnetic nozzle, which permits the jet of liquid metal to be confined without contact with the walls. For example, French Patents Nos. 2 316 026, 2 396 612 and 2 397 251 disclose electro-magnetic devices operating at high frequencies and in which a copper screen is required to obtain the desired confinement.
However, the industrial utilization of such devices, such as in a plant for the atomization production of nickel-based superalloy powders, presents considerable difficulties. French Patent No. 2 457 730 eliminates the copper screen, but since the device operates at low frequency it is necessary in numerous applications to call on high power rates, little compatible with industrialization, as soon as substantial reductions of the liquid metal jet become necessary, particularly in atomization powder production techniques.